Laminated deposition enabled two-dimensional bismuth antimony diodes with a cut-off frequency of ∼50 GHz

As a prototypical topological insulator, the bismuth-antimony (Bi_1-xSb_x) alloy offers both a large spin Hall angle and high carrier mobility. However, preserving their surface-dominated transport properties in the quasi-two-dimensional (quasi-2D) limit remains challenging due to carrier scattering induced by surface roughness. Here, we report a laminated growth strategy followed by a self-diffusion process that enables the fabrication of high-quality quasi-2D Bi_1-xSb_x films with precisely controlled compositions, smooth surfaces, and the preferred (00l) orientation. The effective mass of Bi_1-xSb_x can be effectively manipulated by controlling the Sb content. Notably, the surface roughness of the Bi_0.5Sb_0.5 film (∼0.2 nm) is substantially lower than that of pure Bi (∼7.6 nm) and pure Sb (∼6.3 nm), indicating effective suppression of surface scattering. As a result, the 2D Bi_0.5Sb_0.5 films exhibit a high room-temperature electron mobility of up to 1500 cm² V^-1 s^-1. The optimized surface quality also yields a 20-fold enhancement in mobility relative to previous BiSb films of similar thicknesses. Furthermore, Ti/Bi_0.5Sb_0.5/Pt Schottky diodes fabricated from these films demonstrate a cut-off frequency of 48 GHz, underscoring their potential for high-frequency rectification and advanced 6G communication applications.